The ability of macrophages to release hydrogen peroxide (H202) has been correlated with their ability to kill bacteria and tumor cells. With this observation in mind, a rapid, quantitative microassay for the measurement of H202 has been developed and several human monocyte cell lines that can be induced to produce H202 have been identified. The H202 accumulation is assayed by the horseradish peroxidase-mediated increase in absorbance of phenol red. Utilizing this technology a lymphokine has been found to be produced by some human T cell lines that stimulates increased H202 production in target human monocyte cell lines. Biochemical studies to purify and characterize this factor have been initiated.